Electrical isolation barriers can be identified in many industrial, medical and communication applications where it is necessary to electrically isolate one section of electronic circuitry from another electronic section. In this context, isolation exists between two sections of electronic circuitry if a large magnitude voltage source, typically on the order of one thousand volts or more, connected between any two circuit nodes separated by the barrier causes less than a minimal amount of current flow, typically on the order of ten milliamperes or less, through the voltage source. An electrical isolation barrier must exist, for example, in communication circuitry that connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet.
The Federal Communications Commission (FCC) has determined that residential telecommunications equipment (e.g., solid-state modems) should have surge protection up to a threshold voltage level (e.g., 1500 volts). In particular, the FCC regulations, Part 68, which governs electrical connections to the telephone network in order to prevent network harm, provides that an isolation barrier capable of withstanding 1000 volts rms (root mean square) at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
In order to achieve regulatory compliance, a conventional approach to electrical isolation is by ensuring that there is enough spacing between the telephone network circuits and other circuits referenced to protective earth ground. The components permitted for use across this isolation barrier are limited to transformers, high voltage capacitors, optoisolators, relays and large resistors.
However, it has been determined that in actual field usage, higher surge voltages (e.g. up to 10,000 volts) may occur across this isolation barrier. When such voltages are present, air may begin to ionize anywhere within the enclosed system; dielectric breakdown may occur within the isolation barrier components, including high voltage capacitors. In short, the damage to the system is undeterministic and may require the replacement of the entire system.
One method of reducing the cost of a total system replacement is by use of a pre-determined weak link within and/or before the system. With the weak-link approach, damage due to the high surge voltage is typically known, and thus a field-repair solution is also known. Although the system is rendered inoperable after these large voltage surges, the damage can be fixed easily in the field. Thereafter, a total system replacement is not necessary.
Implementation of the weak link approach can be achieved by using Metal-Varistors (MOVs), Gas Discharge Tubes (GDTs), and/or a spark gap across the barrier. An MOV, however, degrades over the amount of energy it has experienced across its lifetime. Hence, MOVs are unreliable over time. Spark gaps and GDTs both cause a secondary voltage transient event within the system, causing other components to fail. GDTs are more accurate than spark gaps, with regard to the voltage they begin to conduct, but are more expensive than spark gaps.
Thus, there exists an unmet need for a reliable, accurate, and inexpensive system, apparatus, and/or method for providing an alternative to total system replacement when these systems are subjected to high surge voltages.